1. Field of the Invention
The disclosure is related to a line transceiver apparatus, and in particular to a line transceiver apparatus which supports multiple transmission standards.
2. Description of Related Art
Ethernet is currently the most commonly used local area network technology, and the technical standards thereof, such as the technologies related to connections between physical layers and interactions between devices and physical media in open system interconnection (OSI) reference models, have been established in the IEEE 802.3 standard by the Institute of Electrical and Electronics Engineers (IEEE). Currently, twisted pair lines are often used in Ethernet as transmission lines for a communication network, so that electromagnetic interference and crosstalk are reduced.
Since technology related to Ethernet are constantly evolving, many different transmission standards are developed. Currently, some main standards that are widely used include the 10BASE-T standard, the 100BASE-T standard (which is also termed the “Fast Ethernet” standard), and the 1000BASE-T standard (which is also termed the “Gigabit Ethernet” standard). The baseband transmission speed of the above standards are respectively 10 megabits per second (Mbit/s), 100 Mbit/s, and 1000 Mbit/s. There are also other transmission standards for special uses, such as the 10BASE-TE standard which consumes less electricity during data transmission compared to the 10BASE-T standard. “BASE” is the abbreviation for “baseband.” “T” stands for the use of twisted pair lines as transmission lines. The most commonly used twisted pair lines for Ethernet nowadays is the Category 5 line, abbreviated the CAT-5 line. In the above Ethernet baseband transmission standards, the same connector is used, that is, the RJ-45 connector, and standards with higher speeds are mostly compatible with the standards with lower speeds, so that under most circumstances, network equipment adopting different standards with different speeds may be used together.
In Ethernet wired communication technology, line transceivers are used by network equipment such as network interface cards to transmit and receive signals on transmission lines. Since network interface cards are expected to be compatible with Ethernet transmission standards, such as the 10/100/1000BASE-T standards, network interface cards require line transceiver apparatuses which support multiple transmission standards. FIG. 1 is a schematic circuit structure of a conventional line transceiver apparatus 10 which supports multiple transmission standards. Please refer to FIG. 1, a transmission node (such as a network interface card or another piece of network equipment which is not shown) provides a signal Vin to input nodes NVin+ and NVin− of the line transceiver apparatus, and uses the line transceiver apparatus 10 to generate a load signal VL, so as to transmit a signal through a transmission line (here the transmission line is viewed as an external load 130), or the transmission node uses the line transceiver apparatus 10 to receive the load signal VL carried by the transmission line.
The line transceiver apparatus 10 has an operational amplifier (OP-AMP) 110, resistors, 101-104, resistors 111 and 112, and a transformer unit 120 which is able to adjust a turns ratio. The resistors 101-104 are capable of controlling a transmission gain of the OP-AMP 110, wherein the resistors 101 and 102 have an impedance of R1, and the resistors 103 and 104 have an impedance of R2. Therefore, an equation (1) for calculating a transmission gain GainTX is shown below:GainTX=R2/R1  (1)
In the 10BASE-T standard, a voltage amplitude of the load signal VL is about ±2.5 V, whereas in other standards, such as in the 100BASE-T standard and in the 1000BASE-T standard, the load signal VL which supports these standards clearly has a lower voltage amplitude. For example, the voltage amplitude of the load signal VL in the 100BASE-T/1000BASE-T standard is about ±1 V, and the voltage amplitude of the load signal VL in the 10BASE-TE standard is about ±1.5 V to ±1.9 V. Therefore, when the line transceiver apparatus 10 determines the transmission gain GainTX according to the selected transmission standard (such as the 100BASE-T standard), the ratio of the impedance R1 to the impedance R2 is also determined, so that the line transceiver apparatus 10 is unable to transmit or receive the load signal VL under the 10BASE-T standard, and the line transceiver apparatus 10 is hence also unable to fulfill a requirement of supporting multiple transmission standards.
Therefore, in order to support other transmission standards, the line transceiver apparatus 10 uses the transformer unit 120 which is capable of adjusting a turns ratio (such as the transformer unit which has a turns ratio of 1:M, wherein M≧2.) and uses the resistors 111 and 112 to adjust the transmission gain, so that the line transceiver apparatus 10 is able to transmit and receive the load signal VL carried by the transmission line under different transmission standards. However, the transformer unit 120 which has a turns ratio of 1:M is more expensive than transformer units which have a turns ratio of 1:1, and impedance matching can hardly be achieved between an output impedance RTR of the line transceiver apparatus 10 (which the impedance of the of the line transceiver apparatus 10 from an external impedance RL) and the external load RL. Hence it is difficult to achieve the minimum reflective noise and the maximum power transfer efficiency in different transmission standards.